A vehicle having a stop-idling system has been developed in recent years for saving energy. The stop-idling system stops an engine when the vehicle stops for, e.g. waiting for a traffic light to change. When this vehicle restarts the engine with a battery which is obliged to drive a starter, a load having a large capacity, a terminal voltage of the battery drops, accordingly causing malfunction of other electric components installed in the vehicle.
FIG. 6 shows a circuit diagram of vehicle power unit 501 disclosed in Patent Literature 1. Vehicle power unit 501 compensates a voltage drop at the terminal of a battery. In power unit 501, battery 101 is connected to DC/DC converter 103 which includes booster circuit 105 for boosting the terminal voltage of battery 101, and capacitor 107 connected between the terminal of battery 101 and an output terminal of booster circuit 105. Since a series connection between battery 101 and capacitor 107 generates a power supply voltage, DC-DC converter 103 charges capacitor 107 to compensate the voltage drop at the terminal of battery 101.
Vehicle power unit 501 allows capacitor 107 to compensate the voltage drop of the terminal voltage of battery 101, so that the power supply voltage can be stabilized. However, vehicle power unit 501 of the vehicle having a stop-idling system may stabilize the power supply voltage, but provides battery 101 with a heavy load since the restart of the engine requires driving the starter, which needs a large current supplied from battery 101.